U.S. Pat. No. 3,992,099 to Laughlin, incorporated herein by reference, discloses a source discriminator for measuring the angle of arrival and wavelength of beams of radiant energy. Such a system is useful in combat applications to provide information about the source of ranging laser beams and the like. The source discriminator described in the Laughlin patent includes first and second planar radiant energy detectors transversely oriented at a predetermined angle relative to one another to receive radiant energy. Each of the detectors produces an electrical output signal corresponding to radiant energy which impinges on the detector, the output signals are used to determine the angle of arrival and wavelength of the radiant energy impinging on the planar detectors. The detectors have an angular response approximated by the cosine function.
In some applications of source discriminators such as described in the Laughlin patent, it has been found to be desirable to locate the radiant energy detectors at a position remote from the site where the radiant energy is received by the device. In such applications where the detectors are remotely located, it is desirable to provide radiant energy receivers which can receive radiant energy from a wide field of view, preferably as great as 180.degree., and then transmit that radiant energy received to the radiant energy detectors. It is also desirable to provide a bandpass filtering of the radiant energy received. This serves to filter out the background signal of the radiant energy received by the system and increase the signal to noise ratio of the specific wavelengths of light being monitored compared to the background signal. It is also desirable to provide a source discriminator apparatus with a minimum number of parts and a minimum size.
However, it has been found that the combination of these objectives is not easily accomplished in a source discriminator. Moreover, these objectives have not been suitably accomplished by prior art devices. For example, it has been found that traditional means of meeting the objectives of providing a wide angle field of view and increasing the signal to noise ratio (i.e., providing an optical band pass filtering) are mutually limiting.
A typical method of increasing the signal to noise ratio of a radiant energy signal is to pass the signal through a bandpass filter. The bandpass filter blocks most wavelengths of the radiant energy and transmits primarily only the radiant energy having a wavelength corresponding to the predetermined bandpass wavelengths for the particular filter. The median wavelength of the bandpass is termed the central wavelength. A commonly used type of bandpass filter is a dielectric bandpass filter. Dielectric bandpass filters, however, are subject to the phenomena that when the angle of incidence of radiant energy arriving at the filter varies from a direction normal to the surface of the filter, the center wavelength of the radiant energy transmitted by the filter is shifted towards shorter wavelengths. Such wavelength shifting adversely affects the accuracy of a source discriminator in which the bandpass filter is used.
Most simple wide angle lenses which might be suggested for source discriminator applications, however, have a short focal length. The short focal length of the lenses dictates that the lenses also have a focusing cone with a relatively large half-angle.
When such a wide angle lens is used in conjunction with a dielectric bandpass filter, the relatively large half-angle of the lens focusing cone defines the angle of incidence of radiant energy to the filter. The consequent relatively large angle of incidence results in an undesirable center wavelength shift in the transmission through the filter, which in turn results in unacceptable accuracy losses in the source discriminator device.
It has been proposed to use lenses having longer focal lengths with the source discriminator. The longer focal length lenses have a focusing cone with a smaller half-angle which coincides more favorably with the small half-angle of acceptance of the optical band pass filter. However, these longer focal length lenses have a limited field of view and so do not meet the objective of providing a wide field of view. Moreover, even the short focal length wide-angle lenses do not provide a field of view approaching 180.degree..
An additional problem arising in the use of such source discriminator systems is that the photosensors of such systems are frequently sensitive to the polarization of the radiant energy being received. Thus, the signal generated by the photosensor varies depending on the polarization of the light being sensed. To avoid inaccuracies which arise from this sensitivity to polarization, it is desirable that the radiant energy receiving system depolarize the radiant energy received prior to transmitting the radiant energy to photosensors.
The need has thus arisen for an improved wide field of view radiant energy receiver which receives radiant energy over a wide field of view, but which transmits such radiant energy to a filter at an acceptably small half-angle of arrival and which also depolarizes radiant energy received by the system.